Solar cells are known in the art. A solar cell may include, for example, a photoelectric transfer film made up of one or more layers located between a pair of substrates or other layers. These layers may be supported by a glass substrate. Example solar cells are disclosed in U.S. Pat. Nos. 4,510,344, 4,806,436, 6,506,622, and 5,977,477, the disclosures of which are hereby incorporated herein by reference. Substrates in solar cells (or photovoltaic devices) are sometimes made of glass. Glass that is fairly clear in color and highly transmissive to visible light is sometimes desirable in solar cell applications.
Glass raw materials (e.g., silica sand, soda ash, dolomite, and/or limestone) typically include certain impurities such as iron, which is a colorant for glass. The total amount of iron present is expressed herein in terms of Fe2O3 in accordance with standard practice. However, typically, not all iron is in the form of Fe2O3. Instead, iron is usually present in both the ferrous state (Fe2+; expressed herein as FeO, even though all ferrous state iron in the glass may not be in the form of FeO) and the ferric state (Fe3+). Iron in the ferrous state (Fe2+; FeO) is a blue-green colorant, while iron in the ferric state (Fe3+) is a yellow-green colorant. The blue-green colorant of ferrous iron (Fe2+; FeO) is of particular concern when seeking to achieve a fairly clear or neutral colored glass, since as a strong colorant it introduces significant color into the glass. While iron in the ferric state (Fe3+) is also a colorant, it is of less concern when seeking to achieve a glass fairly clear in color since iron in the ferric state tends to be weaker as a colorant than its ferrous state counterpart.
It has been found that the use of a low-iron highly transparent (optionally patterned) glass is advantageous for solar cell applications. The use of the low-iron composition in combination with the patterned surface(s) of the glass substrate(s) has been found to be advantageous with respect to optical properties, thereby leading to increased solar efficiency of a solar cell.
In photovoltaic devices such as solar cells, it is desirable for the glass substrate on the light incident side of the device to have a high total solar (TS) transmission. The higher the total solar (TS) value, the more energy which reaches the semiconductor absorber layer of the photovoltaic device, and the more electrical energy which is generated. Thus, it will be appreciated that low % TS values are undesirable for glass substrates in photovoltaic devices, especially for such glass substrates on the light incident side of such devices. This is because it is generally desirable for the glass substrate on the light incident side of a photovoltaic device to allow as much radiation as possible to pass therethrough so that the photoelectric transfer film (or semiconductor absorbing film) of the device can convert the radiation to as much electrical energy as possible. The less radiation allowed to pass through the glass substrate, the less current generated in the photovoltaic device. For example, the conventional high transmission clear glasses “Regular clear” and “ExtraClear” set forth at the left-hand portion of FIG. 1 have undesirably low % TS values of 84.84% and 88.55% (ISO 9050), respectively.
It would be desirable if a high transmission clear glass could be provided so as to have a higher % TS value than the 84-88% TS Values of the conventional “Regular clear” and “ExtraClear” glasses set forth at the left-hand portion of FIG. 1.
It has been found that ferrous iron (Fe2+; FeO) is of particular concern when seeking to maximize % TS values of glass. This is because the ferrous iron blocks significant amounts of IR radiation and some visible radiation, each of which substantially contribute to % TS. Thus, high % FeO content can lead to undesirably low % TS values in photovoltaic applications and the like.
In the past, antimony (Sb) has been used in glass in an attempt to reduce % FeO in glass and achieve good characteristics. In certain instances, antimony has been added to the glass in the form of antimony trioxide (Sb2O3), sodium antimonite (NaSbO3), and/or sodium pyroantimonate (Sb(Sb2O5)). However, it has been found that antimony is undesirable in certain example instances in that it incompatible with certain float processes (e.g., tin bath). Thus, in certain example embodiments of this invention, the highly oxidized glass is achieved without the need for antimony (which includes antimony oxide), although trace amounts may be present in certain instances.
Another approach to reducing % FeO in a high transmission glass has been to use cerium oxide (CeO2) in the glass. However, the use of substantial amounts of cerium oxide (CeO2) in high transmission glass is undesirable because cerium oxide is both expensive and can lead to undesirable coloration in certain example instances.
One or more of the aforesaid problems may be solved using one or more of the example embodiments of this invention.
In certain example embodiments of this invention, a glass is made so as to be highly transmissive to visible light, to be fairly clear or neutral in color, and to consistently realize high % TS values. High % TS values are particularly desirable for photovoltaic device applications in that high % TS values of the light-incident-side glass substrate permit such photovoltaic devices to generate more electrical energy from incident radiation since more radiation is permitted to reach the semiconductor absorbing film of the device. It has been found that the use of an extremely high batch redox in the glass manufacturing process permits resulting low-ferrous glasses made via the float process to consistently realize a desirable combination of high visible transmission, substantially neutral color, and high total solar (% TS) values. Moreover, in certain example embodiments of this invention, this technique permits these desirable features to be achieved with the use of little or no cerium oxide.
In certain example embodiments of this invention, a soda-lime-silica based glass is made using the float process with an extremely high batch redox. An example batch redox which may be used in making glasses according to certain example embodiments of this invention is from about +26 to +40, more preferably from about +27 to +35, and most preferably from about +28 to +33 (note that these are extremely high batch redox values not typically used in making glass). In making the glass via the float process or the like, the high batch redox value tends to reduce or eliminate the presence of ferrous iron (Fe2+; FeO) in the resulting glass, thereby permitting the glass to have a higher % TS transmission value which may be beneficial in photovoltaic applications. This is advantageous, for example, in that it permits high transmission, neutral color, high % TS glass to be made using raw materials having typical amounts of iron in certain example instances (e.g., from about 0.04 to 0.10% total iron).
In certain example embodiments of this invention, the glass has a total iron content (Fe2O3) of no more than about 0.1%, more preferably from about 0 (or 0.04) to 0.1%, even more preferably from about 0.01 (or 0.04) to 0.08%, and most preferably from about 0.03 (or 0.04) to 0.07%. In certain example embodiments of this invention, the resulting glass may have a % FeO (ferrous iron) of from 0 to 0.0050%, more preferably from 0 to 0.0040, even more preferably from 0 to 0.0030, still more preferably from 0 to 0.0020, and most preferably from 0 to 0.0010, and possibly from 0.0005 to 0.0010 in certain example instances. In certain example embodiments, the resulting glass has a glass redox (different than batch redox) of no greater than 0.08, more preferably no greater than 0.06, still more preferably no greater than 0.04, and even more preferably no greater than 0.03 or 0.02.
The glass substrate may be patterned, or not patterned, in different example embodiments of this invention.
In certain example embodiments, the glass substrate may have fairly clear color that may be slightly yellowish (a positive b* value is indicative of yellowish color), in addition to high visible transmission and high % TS. For example, in certain example embodiments, the glass substrate may be characterized by a visible transmission of at least about 90% (more preferably at least about 91%), a total solar (% TS) value of at least about 90% (more preferably at least about 91%), a transmissive a* color value of from −1.0 to +1.0 (more preferably from −0.5 to +0.5, even more preferably from −0.35 to 0), and a transmissive b* color value of from −0.5 to +1.5 (more preferably from 0 to +1.0, and most preferably from +0.2 to +0.8). These properties may be realized at an example non-limiting reference glass thickness of about 4 mm.
In certain example embodiments of this invention, there is provided a method of making glass comprising:
Ingredientwt. %SiO267-75%Na2O10-20%CaO 5-15%total iron (expressed as Fe2O3)0.001 to 0.1%% FeO  0 to 0.005wherein the glass has visible transmission of at least about 90%, a transmissive a* color value of −1.0 to +1.0, a transmissive b* color value of from −0.50 to +1.5, % TS of at least 89.5%, and wherein the method comprises using a batch redox of from +26 to +40 in making the glass.
In certain example embodiments of this invention, there is provided a glass comprising:
Ingredientwt. %SiO267-75%Na2O10-20%CaO 5-15%total iron (expressed as Fe2O3)<=0.1%% FeO<=0.005glass redox<=0.08antimony oxide0 to less than 0.01%cerium oxide0 to 0.07%wherein the glass has visible transmission of at least 90%, TS transmission of at least 90%; a transmissive a* color value of −1.0 to +1.0, a transmissive b* color value of from −0.5 to +1.5.
In still further example embodiments of this invention, there is provided solar cell comprising: a glass substrate; first and second conductive layers with at least a photoelectric film provided therebetween; wherein the glass substrate is of a composition comprising:
Ingredientwt. %SiO267-75%Na2O10-20%CaO 5-15%total iron (expressed as Fe2O3)<=0.1%% FeO<=0.005glass redox<=0.08antimony oxide0 to less than 0.01%cerium oxide0 to 0.07%wherein the glass substrate has visible transmission of at least 90%, TS transmission of at least 90%; a transmissive a* color value of −1.0 to +1.0, a transmissive b* color value of from −0.5 to +1.5.